Ionization

1
Ionization
2
Measuring Ions

• A beam of charged particles will ionize gas.
• Particle energy E
• Chamber area A
• An applied field will cause ions and electrons to
  separate and move to charged plates.
• Applied voltage V
• Measured current I

I

E
A
-
V
3
Saturation

• Ion electron pairs created will recombine to
  form neutral atoms.
• High field needed to collect all pairs
• V gt V0
• Uniform particle beam creates constant current.
• Saturation current I0

I
I0
V
V0
Ion recombination
Saturation
4
Saturation Current

• A uniform beam is defined by fluence rate and
  energy.
• Intensity is the product
• The number of ions depends on the gas.
• Ionization energy W
• The saturation current is proportional to
  intensity

• Energy per area per time
• The number of ion pairs N is
• The saturation current is

5
Ionization Energy

• W values measure the average energy expended per
  ion pair.
• Electrons uniform with energy
• Protons above 10 keV similar to electrons
• W for heavy ions increases at low energy.
• Excitation instead of ionization

• Gas Wa Wb (eV/ion pair)
• He 43 42
• H2 36 36
• O2 33 31
• CO2 36 33
• CH4 29 27
• C2H4 28 26
• Air 36 34

6
Electrometer

• Typical Problem
• A good electrometer can measure a current of
  10-16 A.
• What is the corresponding rate of energy
  absorption in a parallel-plate ionization chamber
  with W 30 eV/ip?

• Answer
• The energy rate is related to the intensity.
• (10-16 C/s)(30 eV)/(1.6 x 10-19 C) 1.88 x 104
  eV/s
• Equivalent to one 18.8 keV particle per second

7
Smoke Detector

• Many household smoke detectors are ionization
  chambers.
• Electric field from a battery
• 241Am alpha source (.5 mg)
• Smoke interrupts saturation current through
  recombination.

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8
Liquid Argon

• Liquid noble gases can be used in ionization
  chambers.
• Liquid argon, krypton, xenon
• An applied field of 1.1 MV/m used to suppress
  scintillation in liquid Ar.
• Focus on an example from the Dzero
  electromagnetic calorimeter.

• Liquid argon parameters
• Density 1.41 g/cm3
• Boiling point 87 K
• W value 23.6 ev/ion pair

9
Uranium Cell
4.0 mm
2.3 mm
4.3 mm

• Uranium plates are alternated with readout pads.
• Separated by liquid argon
• Readout pads are 5-layer printed circuit boards.
• Outer readout pads
• Inner layer readout wires
• Ground planes to reduce crosstalk
• Resistive coat at 2.5 kV

depleted uranium
liquid Ar gaps
readout pad
10
Shower Production

• Uranium acts as an absorber.
• Density 19.05 g/cm3
• Interaction primarily in uranium
• 4 cm for electromagnetic
• Shower particles ionize liquid argon in the gaps.
• Measured on circuit board pads

incident particle
depleted uranium
11
Sampling Calorimeter

• The energy loss in the uranium is much greater
  than in the argon.
• Ionization is a sample of the particles in the
  shower.
• Readout signal is proportional to a sample of the
  shower energy.
• A sampling calorimeter loses some resolution due
  to statistics.
• Gains in flexibility to construct optimal layer
  thicknesses

12
High Voltage Response
13
Pulse Mode

• Ionization signals are read out as individual
  pulses.
• Cell is a capacitor CD
• Total charge dQ proportional to ionization
• Charge sensitive preamp integrates current pulse
  to get charge.
• Measure as voltage change

CF
iin
vout
CD
14
Pedestal

• Integrating the charge means selecting a sample
  time and initial voltage.
• 2.4 ms
• Subtract the baseline voltage
• The distribution with no input signal is the
  pedestal.
• Asymmetric due to uranium noise

15
Cell Noise

• The pedestal is not constant.
• Variation of the pedestal contributes to
  statistical error.
• Depends on cell capacitance
• High voltage on picks up uranium noise

16
Resolution

• Total energy for a particle is due to a sum of N
  channels.
• Resolution varies with total energy
• Signal variance S2 depends on a number of sources
  of error.
• Statistical channel error s
• Channel crosstalk error c

17
Electromagnetic Calibration

• Electrons of known energy are used to calibrate
  the cells.
• Initial digital counts Aj
• Cell calibration bj
• Tower calibration a
• Offset for other material d
• Compare beam momentum to measured energy for
  various energies.

18
Beam Response
19
Measured Resolution

• Energy resolution is measured as a fraction s/E.
• From mean and standard deviation fit to Gaussian
• Resolution is fit to a quadratic as a function of
  momentum
• Channel-to-channel variation C
• Statistical sampling S
• Energy-independent noise N

• Fit results
• C 0.003 0.003
• S 0.157 0.006
• N 0.29 0.03
• Quoted resolution